In the past many innovative joist solutions have been created to improve the performance of floor joists for residential buildings.
Today house framing in North America is predominantly wood construction. The wood industry provides an I-Wood section that has the predominant market share in North America. The I-Wood joist is typically comprised of solid wood chord members that are adhered to a web that is a laminated chip board type member. Wood joists have a cost advantage over steel because the installation price for wood is less than steel. When housing construction is slow in North America the price for wood products goes very low, so it becomes very hard for steel to compete with wood for non rated floors based on cost.
Steel floor joist construction is gaining popularity now, the Light Steel Frame (LSF) industry has spent the last several years training framers, engineers, architects and contractors about steel framing. Today the market is predominantly cost based; owners and contractors typically build the cheapest way possible. For steel there have been many innovations improving the method of installation and the provision for follow up trades based on using various C-Shape designs. Although there have been considerable improvements, the use of steel joists has not reached a point wherein they are considered a mainstream method for building. In North America I-Wood joists have predominant market share in the framing market because the site laborers are typically trained for building with wood and because the required tools are on hand. For steel joists Standardized Connectors and Snap-in bridging assist with simplifying installation so that steel can be more competitive with wood. Where I-Wood is most competitive, at spans 16 ft and less for non rated floors, up until the present invention steel beam options have not presented cost efficient alternatives.
For fire and acoustic rated floors, I-Wood joists do not perform as well as solid wood joists or steel joists in certain situations. I-Wood joists face increasing criticism from Fire Officials in North America because installed joists have not performed well under real fire conditions. These I-Wood joist floors have caused injuries, and sometimes fatalities to fire fighters in North America every year where they have fallen through the floors when the joists have failed without warning during a fire. This faulty performance has resulted in calls from Fire Officials to sanction the I-Wood joists or improve their structural performance in fire. As can be seen in the FIG. 1 showing a chart titled ‘Compare Results of ULC-S101 (Full-Scale Fire Resistance Tests), the I-Shaped steel joists (sold under the trademark iSpan™) outperformed the I-Wood joists in these fire tests. This better performance is significant to Fire Officials since the failure mode of the I-Shaped steel joist is typical of what they have experienced with solid wood joists in the past. FIG. 2 shows the structural members that were compared. Specifically (a) shows the I-wood joist; (b) the C-shaped steel joist; (c) the I-shaped steel joist sold under the trademark iSpan™; and (d) the solid wood joist. Note in the chart the rapid failure to carry load for the tested I-Wood joist and the more gradual failure of the tested I-Shaped steel joist.
Accordingly, an I-Shaped steel joist that performs well in a fire and is cost competitive would be advantageous for competing in short span wood applications.